Lensless camera acquires images efficiently

A new kind of camera that does not form a physical image has been developed by scientists in the US. Employing just a single sensor and an array of apertures rather than a lens, the device can acquire the image of a scene using fewer measurements than conventional cameras. According to its inventors, it would be particularly good for surveillance operations and could also be adapted for non-visible imaging at wavelengths such as infrared, millimetre-wave and terahertz.

Conventional cameras use a lens to project an image onto photographic film or an array of charge-coupled devices. What Paul Wilford and colleagues of Alcatel-Lucent's Bell Labs in New Jersey have instead done is to use a single point-like sensor to record the intensity of light that has passed through an array of tiny apertures placed between the object and the sensor. The researchers essentially create an image in time instead of space and do so using just a fraction of the measurements needed in normal digital cameras.

One aperture at a time

Each of the apertures in the array can be independently opened and closed using a computer. The simplest way to record an image is to open one aperture at a time, measuring the intensity of the light arriving at the sensor that has bounced off the object and passed through just that aperture. The number of required measurements is therefore the number of apertures, with the process equivalent to building up an image from a pinhole camera one pixel at a time.

Instead of using this simple method, the team uses an established signal-processing technique known as compressive sensing to reduce the amount of data needed to reconstruct an image. Wilford and colleagues do this by generating pseudo-random patterns of open and closed apertures within the array. Rather than measuring the intensity of light passing through one aperture at a time, the combined intensity from all of the apertures (open and closed) is measured. It then generates another pseudo-random pattern and measures the light transmitted by that, and so on.

"The pseudo-random patterns of the apertures make it possible to capture the information from the scene with a reduced number of measurements," explains Wilford.

LCD apertures

The team built its aperture array from a monochrome liquid-crystal display (LCD) comprising some 65,000 squares that can be made either transparent or opaque. The sensor is a single pixel from a photovoltaic sensor that records the intensity of red, blue and green light falling on it. The research group enclosed the array and sensor inside a light-tight box about the size of a small microwave oven and used a computer to generate the aperture patterns and synchronize this pattern generation with the recording of the intensity measurements. These measurements were then converted into images in a separate step.

The researchers imaged a football, a pile of books and a toy sleeping cat, and found that they could generate reasonable images of these objects using just a quarter (or in the case of the football an eighth) of the measurements that would be needed using a conventional camera. They also imaged the books using two sensors in different positions at the same time, showing how multiple sensors can be used either to reduce the time needed to capture an image or to improve its resolution.

According to Wilford, the device has a number of advantages over conventional cameras. The absence of a lens, he says, potentially makes it cheaper, lighter and free of distortions, with the image quality in theory only limited by the resolution of the aperture array. He adds that the device could be used for low-power imaging, since only a fraction of the data is collected at source and no processing is required to compress the image. The camera would be particularly well suited to surveillance, where the aim is to record changes in a scene and not to make detailed images of it. Wilford adds that it might also be used for imaging at different wavelengths where sensors are expensive.

Technical hurdles

However, the team acknowledges that it must overcome a number of technical hurdles before commercializing its device. The main one is speed. Limited by the working frequency of the LCD display and the sensor's response time, one picture takes between several minutes and an hour to acquire. Boosting the frequency will mean making the apertures much smaller if energy consumption is to be kept under control. One option, according to Wilford, is to use microelectromechanical systems (MEMs) – moving devices built from components measuring as little as microns across.

Tiny, fast-moving apertures introduce a new problem – how to collect enough light in each measurement to create an image. A rival team at Rice University in Texas has made a similar device but has used lenses both to focus the light from an object onto an aperture array and then to direct the outgoing light on to a single-pixel sensor. Wilford admits that increasing the signal-to-noise ratio of his group's sensor to the point where it can cope with the aperture speeds needed to produce videos will be "a challenge", but underlines his group's desire to "get away from this idea of creating a physical image". "We face a number of practical issues," he says, "but this is not by any means outside the realm of today's technology."

They mentioned the similarity to pin-hole cameras

Yeah...I had one of these prototypes as a kid....it was called a pinhole camera...bet mine was cheaper

The difference is that your pin-hole camera had one aperture and possibly billions of light detectors (silver halide crystals), where as this one is the opposite: thousands of apertures and a single light detector.

So, yeah, pin-hole cameras are a lot cheaper but they don't work in the same way.

Not sure what its good for...

Sorry I don't see the point (intended)...we have multiple sensor arrays now in the tens (if not hundreds) of millions of pixels..this idea would require us to make aperture arrays of at least (say) a tenth of the same resolution (its somewhat less due to the sampling/mix idea of multiple apertures at once; which can help lower the needed number as the article describes).So what is the gain in imaging capability? Resolution no, speed no, simplicity no, size no, cost (? doubt it very much), I just don't see the direction. Its interesting (but hardly a revelation) as a technique to get an image. Sensors are NOT the expensive part so having just one (3 for color) is not a good approach. The lens can be small if the sensor and light allow (and the speed) - maybe its most useful in non-visible applications where lenses might be tough to make up to a good enough standard...if so then it might be worth something, but making a lens for that wavelength might be better still.Anyway interesting but not really going to be commercial I think.

Quantum camera (?)

Can we use similar single receptor device but with two apertures instead of just one ? The reason for this could be attempt to capture complete image utilizing famous interference wave-particle duality . Sort of "quantum camera" to capture complete image of the object (?)

Surveillance?

In a time where the Governments of this world are already invading on people's privacy in what Barack Obama arrogantly calls a "compromise between privacy and security" - which is simply nonsense - I'm wondering why on earth we want to make even more discreet ways of surveilling people?

Regardless of that, it seems a bit pointless and I don't really see any positive uses for it.

(snip)Regardless of that, it seems a bit pointless and I don't really see any positive uses for it.

Did you read the part about using only one detector? It then could "also be used for imaging at different wavelengths where sensors are expensive".

Imagine a sensor that is expensive AND massive. Using this technique might make it possible to add such a camera to a space probe where weight is a premium. (One sensor with a shutter array instead of a sensor array with a single shutter) ed

Can we use similar single receptor device but with two apertures instead of just one ? The reason for this could be attempt to capture complete image utilizing famous interference wave-particle duality . Sort of "quantum camera" to capture complete image of the object (?)

Comon Mac. Did you read the article?

To speed image acquisition and improve resolution they open multiple apertures and use some post processing to get the image.

The processing does not involve resolving interference patterns. (I don't think the apertures are small enough.)